Transmitting system



y 1929. D. G. LITTLE TRANSMITTING SYSTEM Filed May 15 1926 2 Sheets-Sheet 1 3 QQQN I gil N m mm m R @I on mu J INVENTOR Dorm Id 6. Lif f/e WITNESSES: @a. use

ATTC'JRNEY May D G! LITTLE 1,712,572

TRANSMITTING SYSTEM Filed May 15 1925 2 Sheets-$heet 2 a l i i WITNESSES: I INVENTOR M3- M Dona/a 61 tiff/e Patented May 14, 1929.

" UNITED STATES PATENT OFFICE.

ELECTRIC & MANUFACTURING COMPANY, 'A CORPORATION OF PENNSYLVANIA.

TRANSMITTING SYSTEM.

Application 'filed May 15,

My invention relates to transmitting systems, and it has particular reference to radio transmitting systems.

One object of my invention is to provide a 5 radio transmitting system that will radiate a carrier wave having a constant frequency.

Another object of my invention is to provide, in a radio transmitting system having a piezo-electric-crystal control for the oscillation generator, a high-power amplifier that will not oscillate at undesired frequencies.

Another object'of my invention is to provide, in the power amplifier portion of "a radio transmitting system of the type described, means for preventing feed-back from one unit of the amplifier to a preceding unit.

Another object of my invention is to provide, for the Inaster'oscillator tube of a transmitting system of the type described above, means for controlling the amplitude of the oscillations generated.

Another object of my invention is to pro vide, in a transmitting system of the type described, means for preventing individual amplifier tubes from oscillating at undesired frequencies.

A still further obj ect-of my invention is to provide, in a transmitting system of the type described, means for compensating against energy feedback from the output circuit of an amplifier tube to the input circuit thereof by reason of the inter-element capacities of the tube and the capacity existing between such tube elements and the ground.

Other and more specific objects will be apparent froma consideration ofv the accompanying drawings, and the following description.

Perhaps one of the most efficient types of radio transmitting systems now 1n use, is that known as the master-oscillator, poweramplifier type. In the master-oscillator systems, the output of a low-power oscillating thermionic tube is successively amplified in tubes of increasing power capacity, until the amplitude is sufficient for effective radiation.

It has been found advantageous tov control the frequency of the first tube by piezo-electrio-crystal means, assuring, in this manner, an output at a constant frequency. The er stal-controlled oscillator has one very deci ed disadvantage, however, being proneto'oscillate at a variety of higher harmonics of the fundamental frequency of the crystal.

In my co-pending applicatiomserial No.

1926. Serial no. 109,277.

78,115, filed December. 29, 1925, I have disclosed and claimed a system fortaking ad vantage of the above-specified peculiarity of crystal-controlled oscillators, in order to transmit on ultra-short wave lengths.

It is necessary, when employing a plurality of stages of amplification between the crystalcontrollcd tube and the antenna, or radiating structure, to entirely eliminate the feedback of energy from one stage to a preceding stage. To a certain extent, this feed-back is prevented when a system accordingto the disclosure in my co-pending application is employed, since each stage is tuned to a frequencydiffering from that of the preceding stage. Nevertheless, when high-power tubes are used,the various elements of which have a large capacity-to-ground, it has been found extremely difficult to so neutralize an amplitier that it will be entirely free from any tendency toward self oscillat'ion.

'My present invcntionprovides means for positively and definitely controlling the generation of oscillations in amplifier circuits. To do this, I use, instead of the usual type of neutralizing condenser, a capacitive device which not only exactly duplicates all the inter-electrode capacities of the tubes employed, but also duplicates the capacity-toground of such tubes. Preferably, the device should be evacuated, and provided with three electrodes, and I have found that, by using, as a neutralizing device, a tube of the same type as the tube being neutralized, I can attain substantially perfect results.

For a complete understanding of my invention, reference is made to the accompanying drawings, and to thefollowing description.

. Figure 1 is a diagrammatic view of the circuits involved in a radio transmitting system arranged according to my invention,

Figure 2 is a diagram illustrating the various tube capacities which I have found it necessary to neutralize,

Fig. 3 is a diagram illustrating an alternative method for neutralizing the low-powered, crystal-controlled stage of a radio transmittmg system.

Referring specifically to Fig. 1, a masteroscillator tube 1 having, a grid 2, a filament 3 and a plate 4, is provided with an input circuit comprising a piezo-electric crystal 5 and a radio frequency choke 6. An oscillating circuit composed of an inductor 7 and a condenser 8 is connected between the plate 4 and the filament 3, a large condenser 9 being used as a radio-frequency by-pass. Plate otential for the tube is obtained from a attery 10, a portion 12 of the battery also providing grid-bias potential for the masseries.

ter-oscillator tube land the next tube in the A second amplifier tube 13, of considerably higher power than the oscillator tube 1, is provided, this tube being also provided with the'usual filament 14, plate 15 and grid 16. The grid 16 is coupled to the output circuit 7, 8 of the oscillator tube 1 through a condenser 17, and is supplied with bias potential from the battery 12 through a radio-irequency choke coil 18.

is equal and op through a neutralizing condenser 26, to the grid 16 of the tube 13, the point 27 of connection to the inductor 22 being so chosen that it will, at all times, be at a potential with respect to the central filament tap 25 which osite to that of the point at which the coup ing condenser is connected. The purpose of this will be later explained in connection with Fig.3

A plurality of high-power amplifier tubes 29 are provided each havin a grid 30, a filament 32 and a plate 33. The plates and grids of these'power-amplifier tubes are arranged in parallel, the grids being conductive output circuit of the preceding tube 13. The grids 30 of these tubes are also connected to the filaments through a radio-frequency choke 34, a resistor 35 and 21 kc ing arrangement 36, which will be descri ed in detail later.

An oscillating circuit, comprising a plurality of condensers 37 and an inductor 38, is conductively connected to the plates 33, direct current being fed to the plates through a radio-frequency choke 39. A condenser 40 of large capacity is connected between an intermediate point 41 on theinductor 38 and the filament circuit, toserve as a radio-freguency by-pass. I A plurality of "neutralizing evices- 42 are connected between a point 43 on the inductor 38. and the grids 30 of the power amplifier-tubes 29, in substantially the same manner as is connected the condenser 26 m the preceding stage. These devices 42 are provided with an additional electrode 44,

however, which electrode is connected to the y connected to the inductor 22 in the filament circuit for reasons which will be later ex lained.

The ast amplifier preferably comprises a plurality of extreme y high-power watercooled t-ubes'45, each having a filament 46, plate 47 and grid 48. The plates 47 of these tubes are arranged in parallel, as are also the grids, the latter being connected to the output inductor 38 of the previous stage through a condenser 49, and to the filament circuit through a choke coil 50, a resistor 52, and the keying device 36 previously referred to.

An oscillatory circuit, comprising an inductor 53 and a condenser 54, is provided, care being taken to keep the losses in this circuit as low as possible. The plates 47 of the amplifier tubes 45 are conductively connected to the inductor 53, and the inductor -is also coupled to an antenna 55 through a plurality of coupling condensers 56. An intermediate point 57 on the inductor is connected to the filament circuit through a condenser 58. Direct current for the plates of the amplifier tubes 45 passes through an iron-cored surgechoke 59 which prevents high transient potentials from being fed back to the source of plate potential.

A plurality of three-electrode neutralizing devices 60 are connected between a properly chosen point 62 on .the inductor 53 and the grids 48, in the same manner that the neutralizing devices 42 are arranged in the preceding amplifier stage.

Filament potential for all the tubes thus far described is supplied from a single source (not shown) which may be either a transformer, a motor generator or a storage batter gignaling is accomplished by applying to the grids of the amplifier tubes a potential sufiicient to cause them to block, or stop oscillating. When it is desired that they oscillate, the contact members 63 and 64 of a relay, or keying device 36, are closed, connecting the grids of the amplifier tubes directly to the lament circuit through the resistors 35 and 52. If it is desired to stop the oscillation, the contact members 64 and 65 are closed, which connects the grids to the filament by way of a circuit which may be traced through the resistors 35 52, the added resistor 66, a secondary 67 of a power transformer 68, and the path between a plate 69 and a filament 70 of a rectifier tube 72. The filament 70 of the rectifier is supplied with potential from the This voltage is applied to the grids of tubes 30 and when relay contacts 64 and 65 are closed, and is suflicient to cause them to completely block.

This blocking cannot be accomplished, however, when high-power, water cooled tubes are used, unless feed-back between stages is absolutely neutralized, Such abs0- lute'neutralization is the primary object of my invention, and will now be explained in connection with Fig. 2.

l have established the fact that, not only is it necessary to compensate for the feed back across the grid-plate path of a tube, but the capacit of the tube elements to ground must also be taken into consideration. To stand up under the extremely high voltages existing between the grid and plate of a power amplifier, the insulation of any neutralizing device must be exceptionally good. Such requirements may be met by an evacuated container having therein a plurality of space-separatedelectrodes and so designed as to exactly duplicate the capacity characteristics of the amplifier tube with which it is to be used.

lln Fig. 2, l have illustrated diagrammatically, the various tube capacities that must be compensated. A thermionic device 80, having a filament 81, a grid 82, and a plate 83, has a certain definite capacity between the grid and plate, represented by a condenser 84.

The grid-to-filament capacity is represent-- ed by a condenser 85, the grid-to-ground capacity is represented by another condenser 86, while the filament-to-ground capacity is represented by a condenser 8?. The plate also has a definite capacity with reference to the filament, which is represented by a condenser 88, and the plate to ground capacity 1 thermionic tubes in the system il ustrated in iii is represented by another condenser 99. All of these capacities must be duplicated in the neutralizing device, and, for that reason, l have shown such devices as e uivalent to Fig, l. I

lln practicing my invention, l have actually used thermionic tubesv as neutralizing devices, and have obtained very excellent results therewith.

' lln the operation of a radio transmitting system, such as is shown in Fig. l, the output circuit oi the master-oscillator tube is tuned to the trequency of the piezo-electric crystal. Succeeding output circuits may be tuned to this same frequency, or to successively higher harmonics thereof, as explained in my copending application, Serial No. 78,115, The

harmonic method is perhaps the better of the two, since the teed-back between stages is minimized, and the amphfier is not so prone to oscillate. spontaneously. By repeated a circulatory current oi very amplification,

. large amplitude is made to flow in the last output, or tank circuit, asit'is sometimes coupling this tank cirpower amplifier circuits is neutralized by the devices 60, since the capacities through which feed-back takesplace are exactly compensated. The system will, therefore, radiate no power when the amplifier tubes are blocked,

even though the master-oscillator tube con-v tinues oscillating.

In certain cases it may be desirable to control the amplitude of oscillations generated by the master oscillator, without altering either filament or plate potential, or changing the grid bias. Inasmuch as this tube oscillates solely by virtue of the capacity cou pling through the grid-plate path, it the effectiveness of this coupling is changed the amplitude of the oscillations will be altered.

In Fig. 3, l have illustrated diagrammatically a preferred circuit for the control of oscillations in the master-oscillator tube, corresponding to the tube in Fig. 1. AL thermionic tube 90, having a filament 9i, a grid 92 and a plate 93, is provided with an input circu1t comprising a piezoelectric crystal 9iand a radio-frequency choke coil 95. The filament 91 is supplied with power from a source 96, while a source 9*? supplies the prop er grid bias potential, and plate power is supplied from another source 99.

ltltl lllbi crystal, the-feedback necessary-to sustain oscillations taking place through the grid-- plate capacity of the tube so choosing llltl the pointltll on the inductor 99 to which is connected the neutralizing condenser that it will always be at a potential. equal to, but opposite in phase to the point on the induc tor 99 to which the plate is connected, it is possible to impress on the grid oil the tube an amount of energy suficient to completely stop oscillations. By varying the location on the inductor oi the connection from the neutralizing condenser, and, by varying the size of the condenser, I am enabled to accu rately control the amplitude of oscillations generated,

By my invention, l am enabled to construct a master-oscillator, power-amplifier, radio-transmitting system that has an output at a substantially constant frequency. By

reason of the perfect neutralization, I am llti mil

equipped therewith which will not radiate an those skilled in the art. My invention is accordingly not to be limited except in so far as is necessitated by the prior art and as indicated by the appended claim.

I claim as my invention:

In an amplifying system, an input circuit, an output circuit, a number of vacuum tubes each having a hot cathode, a control electrode and an anode, said cathodes being connected together, to a point of said input circuit and to a point of said output circuit, said control electrodes being connected together and, to a different point of said input circuit,

said anodes being connected-together and to a point in said output circuit at one side of the connection thereof to said cathodes, and a like number of evacuated devices each having three cold electrodes similar in inter-electrode capacities to the respective inter-electrode capacities of the corresponding electrodes of said vacuum tubes, the electrodes corresponding to said hot cathodes being connected together and to the same points of the input and output circuits respettively as said cathodes, the electrodes of said evacuated devices corresponding to said control electrodes being connected together and to the same point of said input circuit as said control electrodes and the electrodes of said evacuated devices corresponding to said anodes being connected together and to a point of the output circuit at the other side of said connection thereof to said cathodes, whereby the inter-electrode capacity of the evacuated devices is suitably connected to act as a new tralizing capacity for the inter-electrode capacity of the vacuum tubes.

In testimony whereof, I have hereunto subscribed my name this 26th day of April, 1926.

DONALD e. LITTLE. 

